A ROLE FOR SUCROSE SYNTHASE IN STARCH SYNTHESIS IN SWEET POTATO

在植物中，蔗糖是能量和碳水化合物的来源。蔗糖在光合作用组织中合成，并被运送到其他植物器官，在那里它被裂解成单糖或衍生物。蔗糖合成酶（SUS）是参与蔗糖分解的酶之一。为了了解甘薯糖分布和代谢的分子机制，我们分离到了编码甘薯SUS （IBSUS）的cDNA （1191 bp）。所得氨基酸与马铃薯SUS同源性最高（91%）。与单子叶序列（水稻和玉米）同源性较低（75%）。已发表的拟南芥SUS序列出乎意料地显示出更低的同源性，只有69%。甘薯块根发育过程中，根中SUS活性增加。酶活性的增强伴随着淀粉含量的增加。SUS在块根中的活性高于叶片、叶柄、茎和纤维根中的活性。在叶片中检测不到酶活性。采用northern bolt杂交技术，对编码SUS基因在甘薯不同组织中的表达进行了研究。在叶柄、茎、纤维根和块根中发现了约2.4 kbp的SUS mRNA信号。该基因在块根中表达量最多。在叶柄和茎中表达量较低，而在叶片中无表达。编码SUS的基因在RNA水平上被蔗糖强诱导。

In plants, sucrose is an energy and carbohydrate source. Sucrose is synthesized in photosynthetically sctive tissues and transported to other plant organs where it is used after cleavage to monosaccharides or derivatives. Sucrose synthase (SUS) is one of the enzymes involved in sucrose breakdown. To understand the molecular mechanisms in sugar distribution and metabolism of sweet potato, we have isolated a cDNA (1,191 bp) encoding SUS of sweet potato (IBSUS). The deduced amino acid of IBSUS was most closely related (91% homology) to potato SUS.Less homology (75%) was found to monocotyledonous sequences (rice and maize). The published Arabidopsis SUS sequence unexpectedly showed an even lower homology value of 69%. The activity of SUS in roots of sweet potato increased during development of tuberous roots. The enhancement of the enzymatic activity was accompanied by an increase in starch content. The activity of SUS in the tuberous roots was higher than that in the other tissues, namely, leaf blades, petioles, stems and fibrous roots. The enzymatic activity was not detectable in the leaf blade. Expression of the gene encoding SUS in different tissues of sweet potato was assessed by northem bolt hybridization. A signal of the SUS mRNA with a size of approximately 2.4 kbp was found in petioles, stems, fibrous roots and tuberous roots. The gene was most abundantly expressed in tuberous roots. Lower expression levels were found in petioles and stems, whereas no expression could be detected in leaf blades. The gene encoding SUS was strongly inducible by sucrose at the RNA level.